1. Field of the Invention
The present invention relates to a diaphragm for speakers (hereinafter simply referred to as “speaker diaphragm”) and a speaker including the speaker diaphragm.
2. Description of the Related Art
A speaker diaphragm for tweeters designed to reproduce a higher range of frequencies is in some cases made from an acoustic diaphragm material having a high elastic modulus so as to improve the frequency characteristic (first design approach). With this acoustic diaphragm material having a high elastic modulus, the frequency at which divided vibration occurs (hereinafter referred to as “divided vibration frequency”) can be shifted to a higher range.
According to the first approach, ceramic materials such as silicon carbide (SiC), carbon graphite, and titanium oxide are used as the acoustic diaphragm material for the speaker diaphragm. Metal materials such as aluminum and titanium are also used.
Another design approach (second design approach) for making the divided vibration frequency higher is to improve the shape and structure of the speaker diaphragm. According to this approach, an elastic modulus substantially as high as that obtained by the first approach can be achieved by improving the shape and the structure of the speaker diaphragm even when an acoustic diaphragm material having a relatively low elastic modulus is used. These approaches have been employed to make the divided vibration frequency higher.
There is also suggested a technique of forming a speaker diaphragm by using a polyimide foam. According to this technique, a polyimide foam, which is a molded block having a predetermined thickness, is compressed under heating using a die (refer to Japanese Unexamined Patent Application Publication No. 2002-374593). As a result, a speaker diaphragm which is light-weight (low density) and has superior environmental resistance, high internal loss (tan δ), high formability, and high shape design flexibility can be obtained. Since the internal loss is high, the divided vibration does not easily occur.